soc/arm/xilinx_zynqmp: clean up use of extern "C"

Remove extern "C" support from files that don't declare symbols subject
to language linkage effects.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

soc/arm/xilinx_zynqmp/soc.h

@@ -10,10 +10,6 @@
 
 #include <misc/util.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #ifndef _ASMLANGUAGE
 
 #include <device.h>
@@ -21,8 +17,4 @@ extern "C" {
 
 #endif /* !_ASMLANGUAGE */
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif /* _BOARD__H_ */